In the last two decades or so, interest in biomass gasification has picked up as means of producing energy from renewable resources to supplement fossil fuels as well as to develop strategy for distributed generation for reasons of meeting energy security needs. This renewed interest has encouraged development of new and improved methods for making biomass gasification efficient and fuel gas generated from these cleaner in terms of its tar content. Biomass typically comprises collectable, plant-derived materials that may be abundant and relevantly inexpensive in comparison to fossil fuels. Additionally, biomass may be potentially convertible to feedstock chemicals or used for electricity generation. Some examples of sources of biomass may be, without limitation, wood, grass, agriculture and farm wastes, manure, waste paper, rice straw or rice husks, corn stores, corn cobs, sorghum stover, poultry litter, sugarcane bagasse, waste resulting from vegetable oil extraction, peanut shells, coconut shells, shredded bark, food waste, urban refuse and municipal solid waste.
The present invention is directed to a reactor vessel in which solid, liquid and gaseous organic wastes such as but not necessarily limited to forestry and agricultural residues, animal wastes, bacterial sludge, sewage sludge, municipal solid waste, food wastes, animal bovine parts, fungal material, industrial solid waste, waste tires, coal washing residue, petroleum coke, oil shale, coal, peat and lignite, waste oil, industrial liquid wastes, residuals from petroleum refining and volatile organic compounds generated by the industrial processes are transformed into gaseous fuels with maximum conversion efficiency while maintaining resultant synthesis fuel gas free of tar and oil. The organic materials of this type commonly referred to as carbonaceous materials include fixed carbon, volatile matter and ash.
Moisture present with all of the carbonaceous is also included in the volatile matter. The primary objective of the transformation is to obtain essentially complete conversion of carbon and volatile matter into synthesis fuel gas, while leaving only ash as solid residue. This transformation of the organic material takes place by combining these organic materials with steam and air or oxygen in a high temperature environment. Gas-solid contact, the temperature and the time allocated for gas-solid contact at a given temperature all play a role in the extent of conversion of the organic material introduced into the reactor vessel. Most of the time, the moisture content of the organic feed material is adequate for the transformation reactions. However, the present invention also includes the benefits of introducing additional moisture to produce uniform quality of the synthesis gas from this apparatus. The present invention does not preclude pre-drying of the organic feed material prior to its introduction into the reactor vessel.
The advantages of converting organic material into synthesis fuel gas over directly combusting the carbonaceous material are quite significant. Direct combustion of carbonaceous materials mentioned above usually results in smoke and discharge of unwarranted polluting compounds to the detriment of human health. Besides, direct combustion results in deposition of tar in the chimneys which poses a fire hazard. In contrast, the synthesis fuel gas, after production and clean-up, contains simple clean burning combustible gases, namely carbon monoxide, hydrogen and some methane along with non-combustible nitrogen, carbon dioxide and water vapor. This synthesis fuel gas is also suitable for fuel use for internal combustion engines.
The ideal device for the transformation of carbonaceous material into synthesis fuel gas would comprise of ability to introduce all types of carbonaceous materials without limitations in reason of its origin, size, and composition and that would also provide ideal mixing between solids present in the device and gas including air and steam that is introduced into the apparatus. There are number of devices that are capable of transforming all sorts of carbonaceous materials into synthetic fuel gas; however, none of them are without limitations.
For example, the bubbling fluidized bed reactors are well known for providing ideal contact between solids and gases; however, these devices lack versatility with respect to handling multiple types and sizes of carbonaceous materials. The operation of fluidized bed device is generally restricted to one particular type and one size of carbonaceous material since any variation in these would upset the delicate balance between fluidization velocity and the size of the carbonaceous material as well as the balance between the composition of the carbonaceous material and amount of reaction gases such as air and steam introduced into the reactor.
Another example of reactor with good contact between solid and gas is the circulating entrained bed reactor. This type of reactor increases contact time between the solids and gases by continuous recirculation of the solids inside the reactor vessel. Again this type of reactor lacks versatility with respect to type and size of the carbonaceous material.
In the small-scale category of the available reactors, common ones are updraft gasifiers, downdraft gasifiers, and cross-draft gasifiers. All of these types of reactors have restrictions with respect to the density and the size of the carbonaceous material they can handle. Besides none of these reactors have ability to provide ideal mixing between solids and gases which is a prerequisite for obtaining maximum conversion of carbonaceous material into synthesis fuel gas. As a result of poor mixing, these reactors lose significant amount of carbon with the solid residue. In comparison to all of the aforementioned devices, the rotary reactor such as kiln is most flexible and versatile in terms of handling vast array of carbonaceous material irrespective, within reasons, of type, composition, and size. The rotary kiln device is also suitable for operating at full load and part load as necessitated by synthesis fuel gas demand or by availability of the carbonaceous material. The primary weakness of the rotary kiln is gas solid mixing without which it is difficult to attain high conversion of carbonaceous fuel into synthesis fuel gas. In a study performed by CPL Industries (Reference 1), it was quite apparent that without allowing provisions for suitable mixing inside the kiln it was not possible to attain high transformations of carbonaceous fuel into synthesis fuel gas. Without adequate mixing between solids and gases, the air and steam has tendency to bypass reaction with solids and instead prefers to react with gases thereby impairing the quality of synthesis fuel gas with respect to its heating value. Moreover the bypassing of air and steam results in lower conversion of carbonaceous material and hence lot of carbon is lost with the solid residue.
The present invention provides an apparatus to introduce air, steam, and other gases which when installed inside of the rotating reactor such as kiln tremendously improves gas solid mixing inside the reactor and thereby assures maximum conversion of carbonaceous material into synthesis fuel gas. With this ability for gas solid mixing and its inherent flexibility with respect to accepting wide array of carbonaceous material irrespective of type, composition, and size; and combined with its ability to operate within large variation of loading of the carbonaceous material, the kiln reactor would become the reactor of choice for distributed power generation for smaller and larger applications.
Some prior attempts to provide improved gas solid mixing in a rotary kiln as well as attempts to improve conversion of carbonaceous material into synthesis fuel gas in rotary kiln by indirect means are mentioned below.